The American kestrel (Falco sparverius) is the smallest and most common falcon in North America. It has about a two to one range in size over subspecies and sex, varying in size from about the weight of a blue jay to a mourning dove. It also ranges to South America, and is a well established species that has evolved seventeen subspecies adapted to different environments and habitats throughout the Americas. It exhibits sexual dimorphism in size (females being moderately larger) and plumage, although both sexes have a rufous back with noticeable barring. Its plumage is colorful and attractive, and juveniles are similar in plumage to adults.

The American kestrel usually hunts in energy conserving fashion by perching and scanning the ground for prey to ambush, though it also hunts from the air. It sometimes hovers in the air with rapid wing beats while homing in on prey. Its diet typically consists of grasshoppers and other insects, lizards, mice, and small birds (e.g. sparrows). This broad diet has contributed to its wide success as a species. It nests in cavities in trees, cliffs, buildings, and other structures. The female lays three to seven eggs, which both sexes help to incubate.

Its breeding range extends from central and western Alaska across northern Canada to Nova Scotia, and south throughout North America, into central Mexico and the Caribbean. It is a local breeder in Central America and is widely distributed throughout South America. Most birds breeding in Canada and the northern United States migrate south in the winter. It is an occasional vagrant to western Europe.

Based on appearance and behavior it was for many years considered a member of the primarily European and African kestrel clade within the genus falco, but recent DNA analysis shows the American kestrel to actually be genetically more closely related to the larger American falcons such as the Aplomado falcon,[2] the Peregrine falcon, and Prairie falcon.[3] Though the species has not been renamed as a result of these genetic analyses, it is not actually a kestrel in the phylogenetic sense. Instead, a process of convergent evolution to fit a similar small prey niche in the ecosystem as the true kestrels has left it with similar physical characteristics and hunting methods.

The American kestrel is a common bird used in falconry, especially by beginners. Though not as strong a flyer as many other larger falcons, proper training and weight control by the falconer allows many American kestrels to become effective hunters of birds in the size range of sparrows and starlings, with occasional success against birds up to approximately twice their own weight.[4]

Under traditional classification, the American kestrel is the smallest raptor in America.[5] The American kestrel is sexually dimorphic, although there is some overlap in plumage coloration between the sexes. The bird ranges from 22 to 31 cm (8.7 to 12.2 in) in length[6] with a wingspan of 51–61 cm (20–24 in). The female kestrel is larger than the male, though less so than larger falcons, being typically about 10% to 15% larger within a subspecies. The more northern subspecies tend to larger sizes, with a large northern female being about twice the size of a small southern male. The male typically weighs 80–143 g (2.8–5.0 oz), and the female 86–165 g (3.0–5.8 oz). In standard measurements, the wing bone is 16–21 cm (6.3–8.3 in) long, the tail is 11–15 cm (4.3–5.9 in) and the tarsus is 3.2–4 cm (1.3–1.6 in).[7][8][9]

Physically, American kestrels are leaner and less muscular than larger falcons.[10] The pectoral flight muscles of the American kestrel make up only about 12% of its body weight, as compared to about 20% for the strongest flying falcons such as the peregrine.[11] The wings are moderately long, fairly narrow, and taper to a point. Their less muscular body type is adapted to energy conserving ambush hunting, rather than spending large amounts of energy consuming time on the wing and getting into long tail-chases of bird prey. For their size, they have strong talons and beaks, and can swiftly dispatch prey. Their lean build and energy conserving strategy allows a lower daily food intake than if they were more strongly muscled, yet with enough strength to commonly take bird prey as large as themselves, and occasionally larger. The success of this body style and hunting strategy is reflected in the high success of the species in densely populating a large range throughout the Americas. The flight of the American kestrel is not so dramatic and swift as more muscular falcons such as Merlins and peregrines, but their efficient adaptation to a broader diet of more available smaller prey, and need for less food per day, has resulted in there being many more of them.

In contrast to many other raptor species, the sexes differ more in plumage than in size. Males have blue-grey wings with black spots and white undersides with black barring. The back is rufous, with barring on the lower half. The belly and flanks are white with black spotting. The tail is also rufous, with a white or rufous tip and a black subterminal band.[12] The back and wings of the female American kestrel are rufous with dark brown barring. The undersides of the females are creamy to buff with heavy brown streaking. The tail is noticeably different from the male's, being rufous in color with numerous narrow dark black bars. Juveniles exhibit coloration patterns similar to the adults'.[12] In both sexes, the head is white with a bluish-grey top. There are also two narrow, vertical black facial markings on each side of the head, while other falcons have one.[13] Two black spots (ocelli) can be found on each side of the white or orangish nape.[14] The function of these spots is debated, but the most commonly accepted theory is that they act as "false eyes", and help to protect the bird from potential attackers.[15]

The American kestrel has three basic vocalizations – the "klee" or "killy", the "whine", and the "chitter."[16] The "klee" is usually delivered as a rapid series – klee, klee, klee, klee when the kestrel is upset or excited. This call is used in a wide variety of situations and is heard from both sexes, but the larger females typically have lower-pitched voices than the males. The "whine" call is primarily associated with feeding, but is also uttered during copulation. The "chitter" is used in activities that involve interaction between male and female birds, including courtship feeding, copulation, and the feeding of nestlings.[17] Nestlings can produce calls similar to those of adults at 16 days old.[18]

Until the sixth edition of the AOU Checklist of North American Birds was published by the American Ornithologists' Union in 1983, the most commonly used name for the American kestrel was the sparrow hawk or sparrowhawk. This was due to a mistaken connection with the Eurasian sparrowhawk in the genus Accipiter. The sixth edition of the AOU Checklist corrected this, officially renaming the bird American kestrel. Several other colloquial names for the kestrel are also in use, including grasshopper hawk, due to its diet, and killy hawk, due to its distinct call.[19]

As noted in the introduction, DNA analysis shows the American kestrel to actually be genetically more closely related to the larger American falcons[2][3] than to the true kestrels. However, based on its physical similarity to the kestrels and the established nature of the name American kestrel, there has been little impetus to change its name. This could change in the future if continued genetic research more precisely determines the evolutionary history of the American kestrel within the genus falco. The entire genus is actually a set of species so closely related that most or all can be hybridized by artificial insemination. Significant natural hybridization of species has occurred in the past during the evolution of this closely related set of species, such that precise evolutionary genetic analysis as to which species are more basal to other species or to the genus as a whole is difficult to render.

The American kestrel's scientific name, Falco sparverius, was given by Carl Linnaeus in his 18th century work Systema Naturae.[20] The genus refers to the falcate, or hooked, shape of the beak, and the specific name means "pertaining to a sparrow", referring to the bird's small size and hunting of sparrows as a typical prey.[19]

Seventeen subspecies of the American kestrel are recognized, generally based upon plumage, size, and vocalizations:[21]

F. s. sparverius, described by Linnaeus in 1758, is the nominate subspecies. It is found in most of the United States, Canada, and Mexico.

F. s. paulus, described by Howe and King in 1902, is found in the Southeast United States, from Louisiana to Florida.

American kestrels are found in a wide variety of habitats, including grasslands, meadows, deserts, and other open to semiopen regions. They can also be found in both urban and suburban areas. A kestrel's habitat must include perches, open space for hunting, and cavities for nesting (whether natural or man-made).[22] The American kestrel is able to live in very diverse conditions, ranging from above the Arctic Circle,[23] to the tropics of Central America, to elevations of over 4,500 m (14,800 ft) in the Andes Mountains.[24] The bird is distributed from northern Canada and Alaska to the southernmost tip of South America, Tierra del Fuego. It is the only kestrel found in the Americas,[25] though as mentioned above this classification is genetically inaccurate. It has occurred as a vagrant in the UK, Denmark, Malta and the Azores.[26]

American kestrels in Canada and the northern United States typically migrate south in the winter, sometimes going as far as Central America and the Caribbean. Birds that breed south of about 35° north latitude are usually year-round residents. Migration also depends on local weather conditions.[27] Wintering kestrels' choice of habitat varies by sex. Females are found in open areas more often than males during the non-breeding season. A common explanation for this behavior is that the larger females arrive at the preferred habitat first and exclude males from their territory.[28]

The American kestrel is not long-lived, with a lifespan of <5 years for wild birds.[29] The oldest banded wild bird was 11 years and 7 months,[30] while captive kestrels can live up to 14–17 years.[29] In a study, humans accounted for 43.2% of 1,355 reported deaths, which included direct killing and roadkills, while predation (including by larger birds of prey) accounted for 2.8%. This statistic is likely biased, however, as reported deaths are usually found near or in areas populated by humans.[29]

American kestrels feed largely on small animals such as grasshoppers, dragonflies, lizards, mice, voles, and small birds. The kestrel has also been reported to have killed snakes, bats,[31] and squirrels.[32] The kestrel is able to maintain high population densities, at least in part because of the broad scope of its diet. The American kestrel's primary mode of hunting is by perching and waiting for prey to come near. The bird is characteristically seen along roadsides or fields perched on objects such as trees, overhead power lines, or fence posts. It also hunts by kiting, hovering in the air with rapid wing beats and scanning the ground for prey. Other hunting techniques include low flight over fields, or chasing insects and birds in the air.[33]

Prey is most often caught on the ground, though occasionally they take birds in flight. Before striking, the kestrel characteristically bobs its head and tail, then makes a direct flight toward the prey to grab it in its talons. Much like the red-tailed hawk, American kestrels conserve energy in a hunt and pick their attacks with care as to position and odds of success.[10] During the breeding season, the bird will carry large prey back to its mate or young. One study found that an American kestrel pair "foraged in ways that minimized the costs of energy acquisition in its particular situation". For example, if the success rate for catching prey decreases significantly in a particular area, the bird will move to a different area.[34]

American kestrels are sexually mature by their first spring.[35] In migratory populations, the males arrive at the breeding ground before females, then the female selects a mate. Pair bonds are strong, often permanent. Pairs usually use previous nesting sites in consecutive years. This gives birds an advantage over younger or invading individuals, as they would already be familiar with the hunting grounds, neighbors, predators, and other features of the site.[36] Males perform elaborate dive displays to advertise their territory and attract a mate. These displays consist of several climbs and dives, with three or four "klee" calls at their peaks. Females are promiscuous for about one to two weeks after their arrival at the nesting site. This is thought to stimulate ovulation.[37] Food transfers from the male to the female occur from about four to five weeks prior to egg laying to one to two weeks after.[38]

American kestrels are cavity nesters, but they are able to adapt to a wide variety of nesting situations. They generally prefer natural cavities (such as in trees) with closed tops and tight fitting entrances, as to provide for maximum protection of the eggs and young.[39] Kestrels occasionally nest in holes created by large woodpeckers,[40] or use the abandoned nests of other birds, such as red-tailed hawks, merlins, and crows.[41] They have been recorded nesting on cliff ledges and building tops, as well as in abandoned cavities in cactuses.[42] American kestrels also commonly utilize nesting boxes.[43]

Three to seven eggs (typically four or five) are laid approximately 24–72 hours apart. The average egg size is 32 mm × 29 mm (1.3 in × 1.1 in), 10% larger than average for birds of its body size. The eggs are white to cream in color with brown or grey splotching. Incubation usually lasts 30 days and is mainly the responsibility of the female, although the male incubates 15–20% of the time. Eggs that are lost are typically replaced in 11–12 days. Hatching takes place over three to four days. Hatchlings are altricial, and are only able to sit up after five days. They grow very quickly, reaching an adult weight after 16–17 days. After 28–31 days, their wings develop and they are able to leave the nest.[44] The young adult kestrels may breed from a year old, and the species has approximately a three to five year life expectancy in the wild.

In ecological terms the reproductive pattern of the American kestrel leans towards a small bird "r-selection" strategy.[45] In r/K selection theory, selective pressures are hypothesised to drive evolution in one of two generalized directions: r- or K-selection.[46]R-selected species are those that place an emphasis on a high growth rate, typically exploiting less-crowded ecological niches, and produce many offspring, each of which has a relatively low probability of surviving to adulthood (i.e., high r, low K). By contrast, K-selected species display traits associated with living at densities close to carrying capacity, and typically are strong competitors in such crowded niches that invest more heavily in fewer offspring, each of which has a relatively high probability of surviving to adulthood (i.e., low r, high K). Between these two extremes the American kestrel is one of the few raptor species that lean towards being r-selected. They are able to breed at one year old, have few non-breeding adults in the population, and have larger broods. Their population growth rate is high relative to larger raptors, which typically lean towards being K-selected.

American kestrels are often useful in scientific studies on animal physiology, and are typically captured using the bal-chatri method or raised in nest boxes for experiments.[47] Kestrel metabolic rate has been found to increase in response to rainfall, and at ambient temperatures below about 25⁰ C. Kestrel metabolic responses to weather and temperature do not vary, however, with sex.[48] Kestrels will increase their oxygen consumption, and therefore their metabolic rate in cold and wet conditions to counteract heat loss.[48][49]

American kestrels' response to environmental stress is measured as blood concentration of corticosterone (CORT,) a hormone produced by the hypothalamic-pituitary-adrenal (HPA) axis that releases stored energy for essential body functions. Extended periods of elevated blood CORT levels may direct metabolic energy away from growth and reproduction.[47] Thus, high levels of traffic disturbance and human development surrounding American kestrel nests are found to increase stress hormones leading to reproductive failure.[50] Among successful nests, however, nestlings do not typically experience a higher stress response to environmental human disturbance, suggesting that they can tolerate a considerable degree of human activity near the nest.[47]

Since American kestrels are carnivores, toxic chemical runoff ingested by their prey can concentrate at high levels in their blood. Wild kestrels are subject to immunomodulation, or an altered immune response, to polybrominated diphenyl ethers (PBDEs), a group of industrial flame retardants that may leach from factories into the environment. When PBDEs accumulate in body tissues of kestrels, the T-cell mediated immune response decreases in efficiency. As a result, kestrels that ingest PBDEs may not respond sufficiently to viruses or other invading microorganisms. In addition, certain PBDEs may suppress growth and development of the spleen and bursa in American kestrels.[51]

The American kestrel is likely the most abundant falcon in North America, although its total population is difficult to quantify, as local populations can change quickly due to resource availability. Count data from the USGS Breeding Bird Survey (BBS) indicate that the North American breeding population is experiencing long-term and gradual but sustained declines, with some regions, such as New England and coastal California, exhibiting more rapid declines.[52][53] Count data from raptor migration corridors also indicate regional population declines and largely corroborate BBS data.[54] The North American population has been estimated at 1.2 million pairs, with the Central and South American populations being as large. A smaller estimate is 236,000 birds wintering in North America. A population increase occurred in the 18th and 19th centuries, probably due to deforestation for agriculture. The resulting pastures provided an ideal habitat for kestrels.[29]

The southeastern U.S. subspecies (Falco sparverius paulus) has declined 82% since 1940 due to a decrease in nest site availability. This decline is a result of longleaf pines being cleared from agricultural fields.[55] Despite this, the American kestrel is classed as Least Concern on the IUCN Red List.[1]

The Peregrine Fund, a leading non-profit organization advancing research and conservation of birds of prey worldwide, launched the American Kestrel Partnership in 2012.[56] The American Kestrel Partnership developed and maintains a web-based network for citizen and professional scientists to enter, manage, and consolidate data from kestrel nest box monitoring programs in the Western Hemisphere. The database is being used by researchers to model and understand relationships between kestrel nesting parameters (e.g., phenology, occupancy, survival, productivity, and nestling weight and exposure to environmental toxins) and environmental factors, such as land use, landscape composition and configuration, climate conditions (e.g., drought), and point sources of environmental toxins. Each breeding season, the American Kestrel Partnership features a live-streaming video feed from the nest box located at The Peregrine Fund's campus in Boise, Idaho.[56]

One important use of American kestrels is in falconry. It is often considered a beginner's bird, though the careful weight control needed to maintain the kestrel's desire to aggressively hunt takes skill. Falconers experienced in extracting the best performance the species is capable of report they are highly reliable on the normal game of sparrows and starlings.[57] More aggressive individuals are sometimes capable of capturing prey up to approximately twice their own body weight, allowing the occasional capture of true game birds such as quail and dove. However, most falconers interested in the reliable taking of such game do prefer larger falcons or hawks. The advantage the American kestrel offers the experienced falconer is its suitability to simple and urban falconry not requiring large tracts of land or the use of hunting dogs. This form of falconry is sometimes referred to as "micro-falconry"[58] or "micro-hawking." The other small raptor species commonly used in micro-falconry are the Merlin, the Sharp-shinned hawk (the smallest accipiter), and the European kestrel (a true kestrel).

Hawking with the American kestrel requires adapting to the strengths and weaknesses of the bird. It is a very small falcon, and even for its size it is less muscular than other small falcons such as the athletic and swift Merlin. It is more adapted to ambush hunting and short chases than to the longer aerial chases larger falcons often adopt. Used within it limits, it is effective. Experienced falconer Matthew Mullenix, author of the book "American Kestrels in Modern Falconry", in an article comparing kestrels to merlins,[10] summed their abilities up as follows:

1. "Kestrels are thin-winged, flat-chested, under-powered and lack acceleration compared to merlins. I say that with much affection for them and with thousands of kestrel kills to prove these are not necessarily damning differences. Comparing a red-tailed hawk to a Harris’ or goshawk will conjure equally negative points of fact, yet we all know how good trained red-tails can be!"

2. "The chief variable to choosing between a kestrel and a merlin may be your hawking land. If you live in open country, or have access at least to good pasture for cattle, a merlin can excel there. If you plan to hunt mostly in town or suburb, and especially if you plan to hawk from a car, I’d recommend the kestrel. The consideration coming in at close second is your intended quarry. To snipe, dove, quail and open-country sparrows, merlins are best suited. For most blackbirds (Icteridea), either falcon can prove effective. Starlings in close are extremely vulnerable to kestrels; but in the open are best prey for merlins. The same holds true for house sparrows, with this exception: sparrows in thick cover are better quarry for kestrels. This is the slip for which I feel the American kestrel is perfect."

3. "Once committed to an attack, trained kestrels tend to follow through to the end. They will stoop into cover, chase birds on foot, bind to quarry twice their size and never let go voluntarily. They have small feet, but as written elsewhere, also have the strongest feet for their size. It is a simple fact that American kestrels hold starlings better than merlins, on average, and will gladly tackle larger quarry than will any jack (male merlin)."

American kestrels do not train so easily as some larger falcons (particularly the Peregrine falcon) in the art of "waiting on" to perform a diving stoop on flushed prey. However, some individual kestrels do master this skill.[59] Falconers sometimes train them to climb to a stooping position with tidbids on kites or balloons that the kestrels learn to climb after. More common hunting techniques are to "slip" them after spotted game from the fist, or to release them from a vehicle window close to spotted quarry. These techniques are more of a natural fit to the kestrel's ambushing methods in the wild.

Falconers using the American kestrel should be alert to protect the falcon from larger predators that may attack the kestrel, particularly if it is distracted on the ground with captured prey. Domestic cats and dogs are the greatest threat to attack the falcon on the ground, but the Cooper's hawk is well known to boldly attack kestrels. This mid-sized American accipiter has sufficient size and strength to carry the kestrel away, though falconers have reported often being successful in recovering the kestrel unharmed by acting quickly to intimidate the larger hawk into releasing the kestrel.[60]

American kestrels are bred in captivity for use in falconry, and are among the easier falcons to breed. They are also sufficiently common that "passage" birds in their first year are relatively easy to trap. Wild caught kestrels "tame down" fairly quickly. They will usually be eating from a falconer's hand the day after capture, be training within a week, and be ready to hunt in three to five weeks. A very tame American kestrel will allow itself to be picked up around the body with one hand, while accepting tidbits from the other hand.[61] Such tameness is very useful when checking or treating the bird for injury or illness.

Migratory raptors native to the United States are protected by the Migratory Bird Treaty Act of 1918, so American kestrels are illegal to possess without a permit (such as a falconry permit) in the United States, Canada, and Mexico.[62]

^Fjeldså, Jon; Krabbe, Niels (1990). Birds of the High Andes: A Manual to the Birds of the Temperate Zone of the Andes and Patagonia, South America. Svendborg, Denmark: Apollo Books. p. 112. ISBN87-88757-16-1.